As a part of an effort to make aircraft, trucks, automobiles, and other filtration application more energy efficient, producers of air filters have searched for ways to produce more efficient and lighter filters for use in vehicles. In the past, higher "Figure of Merit", that is lower pressure drop through a filter, and a higher particulate removal rate have come at the expense of increasing the weight of the filter.
For example, pleated cylindrical glass-fiber air filters have been developed with high removal rates and low pressure drops through the filter. Pleating the fiberglass filter medium makes it possible to expose a greater filter media surface area to an incident flow of fluid in a more compact space than with flat, unpleated filter media. Each pleat has a tip or first-fold facing the incident flow, and a first side and a second side extending downstream from the tip creating a wedge shape pleat. Downstream, the first side of each pleat meets the second side of each adjacent pleat at a second fold. The pleating gives the filter medium an accordion-like appearance.
Pleated filters, however, are subject to two modes of failure, ballooning and bunching. Ballooning occurs when pressure created by the incident fluid flow on the upstream side of the filter pushes the first side of the pleat into contact with the second side of the same pleat, collapsing the pleat's wedge shape. This greatly reduces the performance of the filter by effectively reducing the area available for the fluid flow to exit the filter medium. Bunching is a related problem which also reduces the performance of the filter by effectively reducing its surface area. Bunching occurs when the pressure of the fluid flow squeezes together several of the pleats and unfolds or flattens others.
To prevent ballooning and bunching, several pleat supporting devices have been developed. These include corrugated aluminum separators, hot melt beads, and Pleatloc.TM. dimples. Generally, the corrugated aluminum separators are placed between the first and second sides of the pleats, both on the upstream and downstream sides of the filter medium. Corrugations in the corrugated aluminum separators are oriented parallel to the incoming flow to provide small channel-like passages for the fluid flow to pass through. Unfortunately, glass filters manufactured with corrugated aluminum spacers are heavy and relatively expensive to manufacture.
Hot melt beads are an alternative to corrugated aluminum separators. Hot melt beads are essentially small plastic welds between the first and second sides of the pleats. For each pleat, a plurality of hot melt beads may be periodically spaced along the length of the pleat. Although hot melt beads may provide support to the pleats with less additional weight than corrugated aluminum separators, hot melt beads are still relatively heavy and expensive. Additionally, unlike the corrugated aluminum separators, hot melt beads may be flammable and toxic and, thus, cannot be safely used in aircraft and other applications, such as heating ventilation and air conditioning.
The development of Pleatloc.TM. dimples, disclosed in U.S. Pat. No. 4,452,619, was a significant advancement over the corrugated aluminum separators and the hot melt beads. Pleatloc.TM. dimples are formed in the filter medium itself by flattening the tip of each pleat. For each pleat, a plurality of dimples may be periodically spaced along the length of the pleat. This flattening creates a dimple which extends perpendicularly beyond the sides of the pleat. When the tips of each adjacent pleat are likewise flattened, the dimples provide separation of the adjacent pleats. The second fold, or downstream fold of the pleat, can also have dimples. Fabricating the dimples is relatively inexpensive compared to corrugated aluminum separators and hot melt beads. Because no additional materials are used, the weight of the filter is not increased by use of dimples to space the pleats.
Although dimples are an inexpensive, light, and safe way to prevent pleat bunching or ballooning, they are difficult if not impossible, to effectively fabricate on glass fiber pleats. Pleat-tip fracture is likely when flattening the relatively brittle glass fiber pleats to form the dimples. Pleat-tip fracture is a break in the continuity of the pleat at its tip. With numerous breaks in the glass fiber filter medium, a significant amount of fluid may pass through the tip fractures without being filtered, thus greatly reducing the particle removal efficiency of the filter.
Pleatloc.TM. dimples have been used effectively with less brittle filter media, such as Donaldson Company, Inc.'s Ultra-Web.RTM. filter media. The Ultra-Web.RTM. filter media is produced by depositing a layer of fine fibers on a cellulose medium. The cellulose alone provides little sub-micron filtration, but is used primarily as a support medium for the deposition of the fine fibers. The particle removal efficiency of the Ultra-Web.RTM. filter media can be enhanced by the deposition of additional fine fibers on the cellulose support medium. With the deposition of fine fibers, however, care must be taken during the manufacturing process not to damage the delicate fiber layer.
The need for efficiency is almost nowhere else as great as in the aircraft industry given inherent limitations on space and weight. Flammability is also a concern in the aircraft industry. Relatively expensive and heavy pleated cylindrical fiberglass filters are currently used in aircraft to filter air for the passengers therein because they are very efficient and are constructed from non-flammable materials. These filters have relied on the heavy corrugated aluminum separators to provide pleat separation.